Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and an apparatus for transmitting a reception acknowledgment for a hybrid automatic repeat request of a user equipment in a wireless communication system.
Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) (hereinafter, referred to as LTE) is an influential next-generation wireless communication system standard. In the LTE, when a base station transmits downlink data to a user equipment, the base station first transmits scheduling information on a downlink data channel through a control channel, allocates the downlink data channel according to the scheduling information, and transmits the downlink data through the allocated downlink data channel. The user equipment transmits to the base station reception acknowledgement/non-acknowledgement (ACK/NACK) of the downlink data and the base station may transmit new downlink data or retransmit pretransmitted downlink data according to the ACK/NACK. Such a data transmission scheme is called a hybrid automatic repeat request (HARQ). The HARQ includes synchronous and asynchronous HARQs, and in the case of the synchronous HARQ, synchronization is set between an HARQ process and a subframe, and as a result, new transmission/retransmission of the same HARQ process is performed according to a predetermined timing. On the contrary, in the case of the asynchronous HARQ, the use of the HARQ is directly instructed without the synchronization between the HARQ process and the subframe.
Meanwhile, 3GPP LTE-A (long term evolution-advanced) (hereinafter, referred to as LTE-A) is a next-generation wireless communication system standard developed by improving the LTE. The LTE-A can support low price/low specification user equipments that primarily perform data communications such as meter reading, water level measurement, utilization of a monitoring camera, inventory reporting of a vending machine, and the like. As described above, the low price/low specification user equipments that primarily perform low-capacity data communications are called a machine type communication (MTC) user equipment.
When the MTC user equipment exists in an area supported by the base station, it may be difficult to apply the ACK/NACK timing of the existing HARQ as it is. For example, when the downlink data has a capacity larger than a capacity which the MTC user equipment can process within a specific time, the MTC user equipment may not decode all of the downlink data at the time of transmitting the ACK/NACK.
Further, in spite of not the MTC user equipment, data to be acknowledged/not acknowledged and a time up to the transmission timing of the ACK/NACK are insufficient, and as a result, an ACK/NACK response of the HARQ may not normally be performed during a predetermined time interval.
For example, the LTE-A can support carrier aggregation and support cross carrier scheduling. In the cross carrier scheduling, scheduling information is simultaneously received through a specific cell, but data scheduled by the scheduling information may be received in different cells. In this case, the different cells are not temporally aligned, and as a result, data may be received lately in some cells. In this case, due to an insufficient time between the lately received data and an insufficient time and an ACK/NACK thereof, the lately received data may not normally be decoded.
Alternatively, in the LTE-A, a new control channel allocated to a data domain may be introduced in addition to the existing control channel allocated to a control domain. Since the new control channel may exist in the data domain, a decoding time of data scheduled by the new control channel may be insufficient according to the user equipment.
Therefore, new ACK/NACK transmitting method and apparatus which can be applied to an advanced wireless communication system such as the LTE-A are required.